1. Field of the Invention
The present invention relates to a sensing device, and more particularly, to a sensing device employed in a touch-controlled display device.
2. Description of the Prior Art
Touch-controlled display devices are widely used in various electronic products in the market. When the touch source touches the figures or objects on the touch screen, a sensing device of the touch-controlled display device judges an occurrence of a touch event, and then an internal processing system operates according to a program compiled in advance. Traditionally, sensing devices employed in touch-controlled display devices may be categorized into two different types as shown in FIG. 1 and FIG. 2.
Please refer to FIG. 1 first. The sensing device shown in the figure turns on the transistor M2 mainly by the signal S1 to charge the capacitor C. Then, a photo diode PD generates a leakage current having different intensity in response to different magnitude of the ambient light source, where the ambient light source has an instant magnitude change when there is a touch source approaching, and the leakage current would discharge the capacitor C to change a terminal voltage of the capacitor C. When the signal S2 is raised to a high logic level, the terminal voltage of the capacitor C is raised, and the output voltage Vout is changed correspondingly. Therefore, checking the voltage level of the output voltage Vout can judge whether a touch event occurs. However, regarding this type of sensing device, since the photo diode PD stays in a biased state for a long period of time, the electric charges are accumulated in a PN junction of the photo diode PD or the occurrence probability of defect states is increased, leading to a shifted I-V curve of the photo diode PD. Consequently, it is hard to control the relationship between the discharging extent of the capacitor C and the magnitude of the ambient light source, which affects the accuracy of the touch event judgment.
Moreover, the sensing device shown in FIG. 2 also judges whether a touch event occurs by charging the capacitor C and then discharging the capacitor C. However, this type of sensing device discharges the capacitor C by a photo-transistor M2, and the discharging extent is also affected by the magnitude of the ambient light source. The photo-transistor M2 is biased mainly by fixed voltages VB1 and VB2, and thus continuously operates in the biased state. Besides, the output voltage Vout is generated directly according to the terminal voltage of the capacitor C through the transistor M1, and then the touch event is judged according to the output voltage Vout. Regarding this type of sensing device, the photo-transistor M2 also stays in the biased state for a long period of time. Therefore, the electric charges are accumulated in a junction of the photo-transistor M2 or the occurrence probability of defect states is increased, leading to a shifted I-V curve of the photo-transistor M2. Consequently, it is hard to control the relationship between the discharging extent of the capacitor C and the magnitude of the ambient light source, which affects the accuracy of the touch event judgment. Moreover, the output signal Vout is generated directly according to the terminal voltage of the capacitor C. In the generating process of the output signal Vout, the terminal voltage of the capacitor C may have a slight variation because of the continuous generation of the leakage current. This also affects the accuracy of the touch event judgment. It can be readily known from the two examples mentioned above that the conventional design still has room for improvement.